Having joined the Army with no military experience at the age of 43, he recruited a cavalry unit and gained experience and victories in a succession of battles in East Anglia. Promoted to the General in charge of cavalry in the New Model Army, he trained his men to rapidly regroup after an attack, tactics first employed with great success at the Battle of Naseby. With military success came political power, until he was the leading politician of the time.
The so-called "second civil war", which broke out in 1648 after Charles I's escape from prison, suggested to Cromwell that no compromise with the king would be possible. Many hold Cromwell responsible for the execution of Charles I in January 1649, although there were 59 signatories to the death warrant.
Cromwell's actions made him very unpopular in Scotland and Ireland which, as nominally independent nations, were effectively conquered by English forces. In particular, Cromwell's suppression of the Royalists in Ireland during 1649 still has a strong resonance for many Irish people. The massacre of nearly 3,500 people in Drogheda after its capture -- comprising around 2,700 Royalist soldiers and all the men in the town carrying arms, including civilians, prisoners, and Catholic priests -- is one of the historical memories that has fuelled Irish-English and Catholic-Protestant strife for over three centuries. Cromwell felt justified in ordering the massacre because the city's defenders had continued to fight, in violation of what were then the norm of warfare, after the walls had been breached. See also Trim Castle and River Shannon.
With the king gone, and with it their common cause, Cromwell's unanimous backing dissolved, and the various factions in Parliament became engaged in infighting. In a repeat of the actions the former king had taken that had contributed to civil war, Cromwell eventually dismissed the republican Rump Parliament in 1653 and instead took personal control as, effectively, a military dictator.
In 1657 Cromwell was offered the kingship by a reconstituted parliament, presenting him with a dilemma since he had been instrumental in abolishing the monarchy. After six weeks of deliberation, he turned down the crown, largely because the senior officers in his army threatened to resign if he accepted, but also because it could have placed existing constitutional constraints on his rule. Instead, he was ceremonially installed as Lord Protector at Westminster Abbey, sitting on the former king's throne, practically a coronation and a king in all but name. In fact, the written constitution gave the King's right to issue noble titles to the Protector, and Cromwell used the right. A history of the titles is given in Restoration.
This should have been the end of the story but in 1661 Oliver Cromwell's body was exhumed from Westminster Abbey and was subjected to the ritual of a posthumous execution - on January 30, the same date that Charles I had been executed. He was in fact hung, drawn and quartered. At the end his body was thrown into a pit; his decapitated head was displayed on a pole outside Westminster Abbey until 1685. Since then it changed hands several times before eventually being buried in the grounds of Sidney Sussex College, Cambridge, in 1960.